Aluminum electrolytic capacitors typically include at least one pair of aluminum foil electrodes, etched to increase surface area, and having a surface dielectric oxide layer. The foil electrodes are usually separated by an insulating dielectric spacer material. Typical dielectrics include paper, plastic films, glass cloth, cellulose, perforated Teflon.RTM. or other material that is inert in the electrochemical system. The aluminum foil electrodes and spacers are conventionally rolled into a cylinder that is impregnated with an electrolyte solution, and then this assembly is placed within a container and sealed. This basic electrode structure is known in the art; representative U.S. Patents with figures illustrating such an electrolytic capacitor include U.S. Pat. No. 3,302,071 (FIG. 1) and No. 3,346,782 (FIG. 1).
The function of the electrolyte is to connect the anode and the cathode, both of which have high surface areas and must be kept separated by the dielectric spacer material. The inherent resistance of the electrolyte adds to the resistance of the aluminum electrodes and the resistance of the aluminum oxide dielectric layers. Total resistance for the capacitor is called the equivalent series resistance (ESR), and the electrolyte resistance is a major contributor to the ESR. Optimally, the resistance of the capacitor will be low; unfortunately, however, most of the electrolyte materials which impart high conductivity are inimical to the aluminum oxide dielectric layer on the foil. This causes the capacitance of the aluminum electrolytic capacitor to degrade rapidly.
Thus, a high ESR is disadvantageous to a capacitor. Because the resistance of the dielectric layer and the aluminum electrodes are fixed by the capacitance desired and the requirements of the circuit design, only through modification of the electrolyte can the ESR be decreased.
The resistance of the electrolyte is controlled by three factors. The first is the number of ions dissolved in the electrolyte's solvent. More ions permit greater charge to be transferred in a given amount of time. It is possible for other ions, or ions and the solvent, to form associations which reduce the effective number of ions available to carry charge. Generally, as the solution approaches saturation with respect to a given ion, more ions are associated and fewer are available to carry charge. Thus, the selection of the concentration of the solute is critical; and it generally is made empirically.
The second factor is the mobility of the ions. The mobility is greatly affected by the viscosity of the solvent. Unfortunately, criteria other than viscosity, such as the toxicity of dimethyl formamide, for example, usually govern the selection of a solvent. Regardless of a particular solvent's viscosity, however, smaller ions are normally able to move through it at greater rates.
The third factor is the quantity of charge carried by each ion. Generally, multi-charged ions provide for a more conductive electrolyte.
An ideal electrolyte, therefore, will have many ions with a high charge-to-volume ratio in a low viscosity solvent.
A common electrolyte for aluminum electrolytic capacitors is the borate-glycol system, as discussed in U.S. Pat. No. 4,376,713. However, such electrolytes are disadvantageous in a number of respects, particularly as they are unsuitable for working at temperatures above 85.degree. C. or below -40.degree. C., or at voltages below 25 volts, as further discussed in Canadian Patent No. 694,253. Capacitors typically are required to function over relatively long time periods and over wide temperature ranges while maintaining a reasonably constant capacitance and impedance. Thus, any corrosion of the aluminum oxide layer of an electrode is particularly evident when capacitors are utilized continuously or at elevated temperatures. Various other electrolyte compositions have been proposed in an attempt to increase capacitor life and operating characteristics.
U.S. Pat. No. 3,138,746, for example, discloses a non-corrosive electrolyte for an electrolytic capacitor. The electrolyte includes an ionogen, for example, formic acid that is neutralized or partially neutralized with ammonia (or an amine) to yield, for example, ammonium formate. This ionogen is dissolved in a suitable solvent such as ethylene glycol. The electrolyte additionally includes one or more anion species of the type utilized in the formation of oxide films on the metal electrodes, such as ammonium borate, or certain phosphates or phosphites.
U.S. Pat. No. 3,346,782 discloses a nonaqueous solvent and an ammonium salt of the formula R.sub.1 COONH.sub.2 R.sub.2 R.sub.3 wherein R.sub.1, R.sub.2 and R.sub.3 are selected from the group consisting of hydrogen, straight, branched, substituted, unsubstituted, saturated and unsaturated C1-C7 alkyl groups and mixtures thereof; for example, ammonium formate, ammonium acetate, and ammonium lactate. An example of an intended formulation is ethylene glycol, ammonium formate, formamide and water. An alternative electrolyte is disclosed in U.S. Pat. No. 4,373,177 which utilizes an electrolyte system of mono(di-N-propylammonium)adipate or mono(di-isopropylammonium)adipate as a solute, phosphate salt, and water dissolved in ethylene glycol as a solvent. This is similar to the electrolyte system of U.S. Pat. No. 4,376,713 in which the electrolyte consists of mono-diethylammonium or mono-triethylammonium adipate as a solute, prepared by reacting diethylamine or triethylamine with adipic acid in ethylene glycol.
Another electrolyte system is discussed in U.S. Pat. No. 3,812,039. This patent teaches an electrolyte consisting essentially of an N-methylformamide solvent and at least 1% by weight of an acid maleate solute that consists in turn of equimolar amounts of maleic acid with ammonia, amine or alkali metal. Another approach is illustrated by U.K. patent application No. 2,041,646 which discloses a non-aqueous electrolyte comprising a cis isomer of an assymetrical unsaturated fatty acid molecule and certain salts thereof.
Japanese publication No. 47-29424 discloses an electrolyte for an electrolytic capacitor containing salts of formic acid and adipic acid in combination with ethylene glycol. However, this reference does not teach the use of formic acid and adipic acid in combination. Furthermore, an amino alcohol is also present in the disclosed composition.
U.S. Pat. No. 3,547,423 discloses capacitor electrolytes comprising an organic base of a substituted ammonia compound in an organic acid. This patent is similar to the Japanese reference in that it contemplates the use of alkanolamine salts of organic acids, including adipic acid and formic acid in electrolyte compositions.